A known DC arc furnace comprises a cylindrical furnace enclosure having a circular hearth for containing a melt, and an arcing electrode centrally positioned in the enclosure above the melt and forming an arc therewith when the electrode and melt are placed in circuit with a source of DC of adequate power. The connections are made so that the melt is anodic and the arcing electrode is cathodic. The melt is obtained by charging the furnace with solid metal pieces such as steel scrap, the charge being melted down to form the melt.
As disclosed by the Valchev et al U.S. Pat. No. 3,835,230, Sept. 10, 1974, the anodic power connection with the melt can be made by a melt contact electrode extending through the hearth and which is laterally offset from the arcing electrode, the electric and magnetic forces involved then causing the arc to deflect angularly in a direction away from the anodic contact electrode. By using more than one contact electrode positioned around the periphery of the hearth, the arc can be made to deflect in different directions by selective connection with one or another of the contact electrodes. With a symmetrical distribution of the contact electrodes and with all of them connected anodically to the DC power source the arc can be vertically directed in alignment with the arcing electrode.
When the arc is operated with an angular deflection relative to the vertical arcing electrode, an arc flare is produced causing radiation to the furnace enclosure wall and thus inducing rapid local erosion of the wall.
The U.S. Stenkvist application Ser. No. 594,739, filed July 10, 1975, now U.S. Pat. No. 4,016,355 Apr. 5, 1977, assigned to the assignee of the present application, discloses that such an electric arc furnace using a melt contact electrode laterally offset from the arcing electrode, can be made to operate with the arc in various angular positions by influencing the arc with a magnetic field created by electromagnets on the outside of the furnace. If the electromagnets, which comprise iron cores and electrically powered coils for the cores, are properly designed and supplied with AC current having a frequency below 25 Hz, the magnetic field influencing the arc can be made to rotate with consequent rotation of the arc at an angularity with respect to the arcing electrode. In other words, the arc can be made to rotatively sweep through a downwardly flaring conical path.
In all of these arrangements with the arcing electrode positioned on the axis of the symmetrical furnace enclosure and hearth, it would seem that uniform unconcentrated erosion of the furnace enclosure's wall can be obtained. A vertical arc should radiate symmetrically outwardly in all directions. An angularly deflected arc forming an arc flare involving concentrated radiation against the furnace enclosure wall should also provide uniform wall wear if made to rotate uniformly using a low frequency AC to power the electromagnetic coil or coils providing the arc directional control.
However, it has been found that even under the above circumstances from the start of the melt-down of the solid pieces charged to the end of the melt finishing the arc radiation does not have a uniform wearing or eroding action on the furnace wall.